1. Field of the Invention
The present invention relates to an injection molding unit having a safety device for prevention of overpressure.
2. Description of the Prior Art
Conventionally, there is known an injection molding unit as shown in FIG. 19, which comprises an injection molding machine 1 and an injection mold 2. The injection molding machine 1 is provided with a screw cylinder 4 having a screw 3 accommodated therein, a heater 5 for heating resin within the screw cylinder 4, a hopper 7 for feeding powder resin 6 into the screw cylinder 4, a hydraulic device 8 for moving the screw 3 forward and backward, and an unshown rotation driving unit for rotating the screw 3 forwardly and reversely. A nozzle 9 is formed at the tip of the screw cylinder 4. The hydraulic device 8 has a hydraulic cylinder 10, and a piston rod 12 integrally coupled with a piston 11 of the hydraulic cylinder 10 serves as the shaft of the screw 3.
To the hydraulic cylinder 10 are connected oil passages 16, 17 provided with selector valves 13, 14, and a throttling valve 15 in such a way that the oil passages 16, 17 communicate with spaces to the front and rear of the piston 11. The oil passage 16 leads to an oil reservoir 18, while the oil passage 17 is connected with an oil pump 19. Another oil passage 20 is also provided between the selector valves 13 and 14. With this arrangement, pressure oil is fed from the oil pump 19 to the rear of the piston 11 and discharged from the front of the piston 11 to the oil reservoir 18, and vice versa, by appropriately actuating the selector valves 13, 14, whereby the screw 3 can be moving forward and backward along with the piston 11.
The injection mold 2 is composed of a cavity mold 21 and a core mold 22, between which is formed a hollow 24 for molding of an injection molded product 23 (in the drawing, reference number "24" is parenthesized beside "23" because it appears therein, overlap with the injection molded product 23). Within the wall portion forming this hollow 24, an unshown cooling water passage is formed. Further, the cavity mold 21 is provided with a resin injection port 27 which communicates with the hollow 24 via a sprue 25 and a runner 26. The nozzle 9 is to be urged against the resin injection port 27.
The injection molding machine 1 and the injection mold 2 are arranged so as to be movable in the axial direction of the screw 3 relative to each other by a mechanism which is not shown.
In this arrangement, the powder resin 6 fed from the hopper 7 into the screw cylinder 4 is transferred toward the nozzle 9 by rotation of the screw 3 and moved to an intermediate point within the screw cylinder 4 and heated by the heater 5, thereby being plasticized into a molten resin state.
In the injection molding process, the injection molding machine 1 and the injection mold 2 are moved relative to each other so that the nozzle 9 is urged into contact with the resin injection port 27 of the injection mold 2 that has been closed, and then pressure oil is fed to the rear of the piston 11 to expand the hydraulic cylinder 10, thereby moving the screw 3 deep into the screw cylinder 4, or toward the nozzle 9. As a result, the molten resin that has accumulated at the tip of the screw 3 is injected from the nozzle 9 to the hollow 24 sequentially through the resin injection port 27, the sprue 25, and the runner 26, and is thus filled into the hollow 24.
The molten resin filled in to the hollow 24 as described above is cooled by the injection mold 2 so as to solidify upon the lapse of a predetermined period. In order to shorten the cooling period, the resin in the hollow 24 is adapted to be cooled by the aforementioned cooling water passage formed in the injection mold 2.
Also, in order to fill the molten resin from the nozzle 9 into the hollow 24 via the sprue 25 and the runner 26, the resin that has accumulated at the tip of the screw 3 needs to be pressurized and injected toward the hollow 24. This pressure to be applied to the resin is referred to as injection pressure. Although normally the injection pressure, if referred to merely as it is, often means a hydraulic pressure introduced to the hydraulic cylinder 10, this hydraulic pressure is represented herein by an injection hydraulic pressure P.sub.OI for the sake of avoiding confusion. What is important is the pressure that acts on the resin, and this pressure changes according to the location of the resin. Thus, a resin pressure at the tip of the screw 3 is represented as P.sub.SC, that at the entrance of the injection mold 2 is P.sub.C1 and that at the terminal end of the hollow 24 is P.sub.C2.
In the conventional injection molding, the injection hydraulic pressure P.sub.OI is normally 80-140 at, whereas the effective area of the piston 11 is about ten times that of the screw 3, so that the resin pressure P.sub.SC at the time of injection is 800-1400 at. The resin pressures P.sub.C1 and P.sub.C2 at that time, although variable in the state of pressure transfer from the tip of the screw 3 into the injection mold 2, depending on the configuration of the hollow 24, can be estimated to be 400-500 at.
The wall surface of the hollow 24 of the injection mold 2 is subject to a load represented by the product of the area of the pressured portion of the wall surface and the resin pressure. For example, if the injection molded product is a TV cabinet with a 60 cm.times.20 cm top surface, on the condition that values of the resin pressures P.sub.C1 and P.sub.C2 are each 400 at., the load that acts on the overall wall surface that forms the top surface of the hollow 24 is as much as 480 t.
On this account, the injection mold 2 is required to have a strength such as to withstand this load, which would result in a great mold thickness. In particular, the cavity mold 21, since it receives internal pressure in a direction such that it spreads outward, is generally manufactured by carving the inside of a steel ingot to obtain enough strength to withstand the internal pressure. As a result, the injection mold 2 would be expensive and take a long time to manufacture. Therefore, if excessive pressure acts on the resin, for example due to a malfunction of the hydraulic device or erroneous operation of the machine, such that the injection mold 2 is damaged, which is exemplified by a crack 28 occurring on the cavity mold 21 as shown in FIG. 20, then not only a great amount of expense will be involved in manufacturing a new injection mold 2, but the injection molding work will also be interrupted for a long time period, resulting in a great economic loss.
To avoid to occurrence of such a situation, it has conventionally been the case that the injection molding unit is provided with a safety device which measures the resin pressure in the injection molding process by an electrical method, and works so as to automatically stop the machine if the measured pressure exceeds a predetermined value, thereby preventing any damage to the injection mold due to excessive pressure.
In the above-described conventional injection molding unit, the safety device used to prevent the injection mold from damage due to excessive pressure is adapted to measure the pressure electrically and indirectly, and thus is not satisfactory in reliability. In other words, when a malfunction arises in the electrical circuit of the safety device, or when electricity is not properly conducted to the electrical circuit, the safety device will not operate, and is one problem.
Also, in the safety device employing an electrical circuit as described above, when the sensor has detected an abnormal pressure, there will be a time lag after the abnormal pressure has already been applied to the injection mold until the pressurization is actually stopped. This accounts for another problem, in that the safety device is insufficient from the viewpoint of preventing breakage of the injection mold.